Abstract: The invention relates to a method of, and apparatus for, determining the clearance between wear surfaces of a reciprocating piston engine and the surrounding cylindrical bore of the engine block, without need for physical contact with the piston. As particularly distinguished from the prior art, the present invention employs an energy wave source, collimated to impinge against selectable portions of the piston head either during its reciprocation, or at rest, in the so-called combustion or compression chamber between the moving piston and the engine cylinder. The energy source may be optical, electro-optical, acoustico-optical, or electromagnetic, generating continuous waves or high frequency pulses, or which are capable of reflecting such energy back to a plurality of detectors Such detectors are capable of measuring exceptionally high frequency (short wave lengths) signals to measure time (and hence distance) from the source to the piston head back to the detector.